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<h4 class="subsection" id="Self_002dReferential-Macros-1"><span>3.10.5 Self-Referential Macros<a class="copiable-link" href="#Self_002dReferential-Macros-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-self_002dreference"></a>

<p>A <em class="dfn">self-referential</em> macro is one whose name appears in its
definition.  Recall that all macro definitions are rescanned for more
macros to replace.  If the self-reference were considered a use of the
macro, it would produce an infinitely large expansion.  To prevent this,
the self-reference is not considered a macro call.  It is passed into
the preprocessor output unchanged.  Consider an example:
</p>
<div class="example smallexample">
<pre class="example-preformatted">#define foo (4 + foo)
</pre></div>

<p>where <code class="code">foo</code> is also a variable in your program.
</p>
<p>Following the ordinary rules, each reference to <code class="code">foo</code> will expand
into <code class="code">(4 + foo)</code>; then this will be rescanned and will expand into
<code class="code">(4 + (4 + foo))</code>; and so on until the computer runs out of memory.
</p>
<p>The self-reference rule cuts this process short after one step, at
<code class="code">(4 + foo)</code>.  Therefore, this macro definition has the possibly
useful effect of causing the program to add 4 to the value of <code class="code">foo</code>
wherever <code class="code">foo</code> is referred to.
</p>
<p>In most cases, it is a bad idea to take advantage of this feature.  A
person reading the program who sees that <code class="code">foo</code> is a variable will
not expect that it is a macro as well.  The reader will come across the
identifier <code class="code">foo</code> in the program and think its value should be that
of the variable <code class="code">foo</code>, whereas in fact the value is four greater.
</p>
<p>One common, useful use of self-reference is to create a macro which
expands to itself.  If you write
</p>
<div class="example smallexample">
<pre class="example-preformatted">#define EPERM EPERM
</pre></div>

<p>then the macro <code class="code">EPERM</code> expands to <code class="code">EPERM</code>.  Effectively, it is
left alone by the preprocessor whenever it&rsquo;s used in running text.  You
can tell that it&rsquo;s a macro with &lsquo;<samp class="samp">#ifdef</samp>&rsquo;.  You might do this if you
want to define numeric constants with an <code class="code">enum</code>, but have
&lsquo;<samp class="samp">#ifdef</samp>&rsquo; be true for each constant.
</p>
<p>If a macro <code class="code">x</code> expands to use a macro <code class="code">y</code>, and the expansion of
<code class="code">y</code> refers to the macro <code class="code">x</code>, that is an <em class="dfn">indirect
self-reference</em> of <code class="code">x</code>.  <code class="code">x</code> is not expanded in this case
either.  Thus, if we have
</p>
<div class="example smallexample">
<pre class="example-preformatted">#define x (4 + y)
#define y (2 * x)
</pre></div>

<p>then <code class="code">x</code> and <code class="code">y</code> expand as follows:
</p>
<div class="example smallexample">
<div class="group"><pre class="example-preformatted">x    &rarr; (4 + y)
     &rarr; (4 + (2 * x))

y    &rarr; (2 * x)
     &rarr; (2 * (4 + y))
</pre></div></div>

<p>Each macro is expanded when it appears in the definition of the other
macro, but not when it indirectly appears in its own definition.
</p>
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